High-frequency electrical transmission line



March 9, 1948. w. w. SALISBURY HIGH-FREQUENCY ELECTRICAL TRANSMISSION LINE Filed Dec. 7, 1942 INVENTOR NFIELD W. SALISBURY Patented Mar. 9, 1948 HIGH-FREQUENCY ELECTRICAL TRANSMISSION LINE Winfield W. Salisbury, Arlington, Mass., assignor to the United States of America, as represented by the Secretary of the Navy Application December 7, 1942, Serial No. 468,139

7 Claims. 1

This invention relates to electrical coaxial transmission lines and especially to those lines in which the central conductor is separated from the outer conductor tube by diiierent insulating mediums at diiierent points along the length of the line.

One of the objects of the invention is to provide an electrical coaxial line having diiierent insulating mediums separating the inner and outer conductors at diiierent points along the line in which the parts are so dimensioned that standing waves caused by reflection from the juncture between insulating mediums are prevented or greatly reduced.

Another object of the invention is to provide an electrical coaxial line having a central conductor supported by spaced insulating beads in which standing waves are prevented or greatly reduced.

Another object of the invention is to provide an electrical coaxial transmission line hav ng a central conduuctor supported by spaced insulating beads in which the beads ar prevented from slipping along one or both of the conductors and may be maintained in accurate spaced relation.

Other objects of the invention and objects relating to the construction and assembly of the various parts will be apparent as the description of the invention proceeds.

One embodiment of the invention has been illustrated in the accompanying drawings in which:

Fig. 1 is a sectional elevational view of a portion of a transmission line embodying the invention;

Fig. 2 is a transverse section taken on the line 2-2 of Fig. 1 through the transmission line at one of the insulators, and

Figs. 3, 4 and 5 are sectional elevational views, similar to Fig, 1, of modified forms of the invention.

In coaxial transmission lines in which the central conductor is supported by insulating beads at spaced intervals along the line, the difference between the dielectric constant of the material of the beads and the dielectric constant of the material in the space separating the beads will cause, unless special precautions are taken, a reflection of energy to take place at each surface of a bead which produces standing waves along the line. I have found that these standing waves may be entirely eliminated or greatly reduced if the characteristic impedance of the line at a section of the line where the bead is the dielectric is made substantially equal to the characteristic impedance of the line at a section between the beads. These characteristic impedances may be made equal by providing a change in diameter of one or both of the conductors at the point where the dielectrics change from One medium to the other.

In Figs. 1 and 2 I have shown a section of a portion of a coaxial line in which the tube or outer conductor It! is of metal and has a circular cross section. The inner conductor II is positioned concentrically Within the tube Ill and is spaced therefrom by means of the insulating beads l2 which fit around the conductor H. The conductor II is reduced in diameter at the portion l3 where the bead surrounds it, forming shoulders M which are perpendicular to the axis of the conductor.

In Fig. 3 is shown a modified form of the construction of Figs, 1 and 2 in which the outer conductor 15 is increased in diameter at l6. where it surrounds the beads II, the inner conductor it having the same diameter throughout.

In some cases I may even use a combination of Figs. 1 and 3, increasing the diameter of the outer conductor and decreasing the diameter of the inner conductor at the place where the bead occurs. An example of this construction is best illustrated in Fig. 5. As this figure combines the principles set forth by Figs. 1 and 3, the reference numerals applied to Fig. 5 correspond to those designating similar elements in Figs. 1 and 3.

As indicated in Figs. 1, 3 and 5, a equals the diameter of the central conductor in the space between the beads, 1) equals the internal diameter of the outer tube at the same place. a equals the diameter of the inner conductor at the portion where the bead surrounds it. and 2) equals the inner diameter of the outer tube at the section occupied by the bead.

In order to obtain the proper diameters for the conductors at the points having the difierent dielectric mediums along the line. I start with the fact that the characteristic impedance. Z0, at a point between the beads should be equal to the characteristic impedance. Z0, where the bead is the dielectric. The characteristic impedance at the section of the line between the beads may be given by the equation:

138 b Z0' gic 5 where e is the dielectric constant at that section of the line.

The characteristic impedance for a section of the line where a difierent. insulating medium enemies separates the inner and outer conductors, as for instance, at a section where. the bead is the dielectric, may be shown by the equation:

where e' is the dielectric constant of thenew insulating medium.

If then, one characteristic impedance is put equal to the other, the following equation may be written:

Where the coaxial line merely consists of the insulating beads with air at atmospheric pressure separating them, the dielectric constantewill equal one, and the following equationmay be written:

If it is desired to have an outer conductor of the same diameter all along the .line, as is shown in Fig, 1, then b will equal b. and, knowingthe dielectric constant e of the bead and assum ng a va u a for the diameterof the inner conductor, then the diameter a of that conductor may be determined. Similarly, if a and a are assumed equal, as in Fig. 3, then values for b and b will not be the same and maybe obtained from the Equation 6. In a. similar manner the dimensions may be calculated in cases where both inner and outer conductors are changed in diameter as above disclosed in connection with Fi 5.

Coaxial lines, however, are often pressurized, or fil ed with gas. or some other dielectric may be u ed in place of the air between the beads l2 of Fig; l and ll of Fig. 3. In such a case the value of would be greater than one and the dime sions of the parts would be represented by the Equation 5 written above. As long. as the equation is sa isfied or substantial y satisfied the beneficial results of the invention will be obtained.

Any suitable. type of insulating material may be used for the beads..l2, one having a low dielectric constant being preferred. Polystyrene is especially adapted for the purpose, having a dielectric constant in the neighborhoodof 2.4. For convenience in putting the beads on inthe construction of Fig, 1, I have shown them split into two sections which fit around the reduced portion of the inner conductor, However, it will be understood that the beads may be made in one piece. if desired, might be molded onto the conductor, or the conductor may be made in sections, to fit together at the' oint where the bead occurs, as by screwing-one section into another.

In Fig, 3 the coaxial line. may be assembled by providing the outer tubing H5 in sections, as indicated, and resistance welding the sections together in a butt joint over the beads; or the sections may be screwed together, or soldered, or attached in any other'desired manner, or the outer conductor may be made in one piece and the beads made telescopic to expand into the enlarged portions of the outer conductor when the central conductor is inserted.

In using the construction of Figs. 1 and 2 it may be desirable to anchor the tubing with respect to the beads and thus prevent slipping of the beads with respect to both conductors. In Fig. 4 is shown one manner in which this may be done. Here, the bead I9 is similar to the bead i2 of Figs. 1 and 2 except that it has a groove Zllaround the circumference into which the wall of the-outer conductor 2| may be rolled, as indicated at 22,

The length of the beads should be small with respect to the wave length for which the line is to be'used, since the losses in the dielectric are greater than the losses in free space. The degree of accuracy necessary in manufacturing this line depends on the standing wave requirement of the associated apparatus, and in some cases the tolerances maybe quite large. However, if the length ofthe head is increased, the required accuracy-is greater for a given standing waveratio.

It will be seen from the-above that I have provided a coaxial line in which the central conductor is separated from the outer conductor by means of different insulating mediums and in which standing waves have been eliminated or substantially'reduced by changing the spacing between the conductors at the point where one insulatingmedium leaves oh? and another begins. The'parts of the line are not diflicult to manufacture and assemble. I the constructions of Figs. 1, 3 and 5 the beads are anchored to prevent them from slipping along the line when the parts are assembled or after the line has been manufactured. In some casesthe arrangement of Fig. 4 may be preferred'in which the beads are preVented from slipping with respect to both conductors. Thisis especially desirable where the lineis to be bent,

Modifications may be. made in the invention without departing from the spirit thereof and I do not desire therefore to limit myself towhat has been shown and described except as such limitations occur in the appended claims;

WhatI desire to claimandsecure by Letters Patent is:

1. An electrical coaxial transmission line comprising, in combination, a metal tube, a conductorextending concentrically within said tube, a plurality of cylindrical insulator members arranged at spaced intervals within said tube to support said central conductor, the diameter of said central conductor being reduced and the diameter of said metal tube being increased at eachof said insulators to an extent which will make the characteristic, impedance of the line where one of said insulators is the dielectric substantially equal to the characteristic impedance of the line at a section between two of said insulators.

2. An electrical coaxial transmission lin comprising, in combination, a metal tube, a conductor of circular cross section extending through said tube concentrically therewith, a plurality of circumferential grooves at spaced intervals along said conductor, each of said'grooves having shoulders perpendicular to the axis of said conductor, a bead surrounding said conductor at each of said grooves, said head having a length sufiicient to fill the space between said shoulders and having a diameter sufficient to support said conductor concentrically within said tube, the diameter of said conductor at said grooves being such that the characteristic impedance of said line at one of said grooves is substantially equal to the characteristic impedance of said line at a point removed from said groove, said grooves providing rigid mechanical support for said beads.

3. An electrical coaxial transmission line comprising, in combination, a metal tube, a conductor of smaller cross section than said tube spaced within said tube concentrically therewith, and a plurality of dielectric beads surrounding said central conductor at spaced intervals along said line to support said conductors concentrically within said tube, the cross sections of both said metal tube and said conductor at each bead being so modified that the characteristic impedance of said line at a section containing one of said heads will be substantially equal to the characteristic impedance of said line at a section not occupied by a bead, said metal tube and said concentrically spaced conductor providing mechanical support for said beads at points of modified cross section, whereby axial displacement of said beads is prevented.

4. An electrical coaxial transmission line comprising, in combination, a metal tube, a circular conductor extending through said tube concentrically therewith, a plurality of circumferentially increased diameter sections at spaced intervals along said metal tube, each of said increased diameter sections having shoulders perpendicular to the axis of said tube, a bead surrounding said conductor at each of said sections, said bead having a length suificient to fill the space between said shoulders and having a diameter sufiicient to support said conductor concentrically within said tube, the diameter of said metal tube at each of said increased diameter sections being such that the characteristic impedance of said line at one of said beads is substantially equal to the characteristic impedance of said line at a point removed from said bead, said beads being rigidly positioned within said line by said metal tube at said increased diameter sections, whereby axial displacement of said beads is prevented.

5. An electrical coaxial transmission line comprising, in combination, a metal tube, a circular conductor extending through said tube concentrically therewith, a plurality of circumferentially increased diameter sections at spaced intervals along said metal tube, each of said increased diameter sections having shoulders perpendicular to the axis of said tube, a plurality of circumferential grooves in said circular conductor, said grooves having shoulders perpendicular to the axis of said conductor and being positioned at intervals along said line to correspond with said increased diameter sections in said metal tube, an insulating bead surrounding said conductor at each of said grooves and extending into the corresponding increased diameter section, the axial distance between the shoulders of each of said grooves and each of said increased diameter sections being equal to the axial length of each of said beads, the diameters of said conductor and said metal tube at each of said beads being such that the characteristic impedance of said line at one of said beads is substantially equal to the characteristic impedance of said line at a point removed from said head, said increased diameter sections and said grooves rigidly positioning said beads whereby axial displacement thereof with respect to said metal tube and said conductor is substantially prevented,

6. A coaxial transmission line comprising, in combination, a substantially cylindrical outer conductor and an inner conductor of substantially circular cross section extending through said outer conductor concentrically therewith, a plurality of insulating beads surrounding said conductor at spaced intervals along said line and serving to space said inner and outer conductors, a groove in said inner conductor at each of said beads, said grooves having shoulders perpendicular to the axis of said inner conductor, the axial length of said bead corresponding to the distance between said shoulders, each of said grooves thereby rigidly positioning one of said insulating beads with respect to said inner conductor, a circumferential groove on the outer surface of each of said insulating beads, said outer conductor being rolled into each of said circumferential grooves thereby rigidly positioning said beads with respect to said outer conductor, the diameter of said inner conductor at each of said beads being such that the characteristic impedance of said coaxial line at one of said beads is substantially equal to the characteristic impedance of said line at a point removed from said groove.

7. An electrical transmission line comprising, in combination, an outer conductor of hollow construction, an inner conductor within said outer conductor, a plurality of spaced insulators for supporting said inner conductor within said outer conductor, the dimension of at least one of said conductors being altered at each of said insulators to provide shoulders abutting surfaces of the corresponding insulator extending substantially perpendicular to said conductors for preventing longitudinal movement of said insulators with respect to said conductors, the extent of the alteration of said dimension being such as to equalize the transmission line characteristic impedance at an insulator with the characteristic impedance of said transmission line in the space between insulators.

WINFIELD W. SALISBURY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,165,961 Cork July 11, 1939 2,187,584 Gothe Jan. 16, 1940 2,197,027 Witt Apr. 16, 1940 2,210,400 Fischer Aug, 6, 1940 2,267,371 Buschbeck Dec. 23, 1941 

